Abandoned oil and gas (AOG) wells can provide pathways for the migration of
subsurface fluids such as methane to the atmosphere. Recent estimates from direct
measurements of methane fluxes show that methane emissions from AOG wells are
in the range of 4-7% of Pennsylvania’s anthropogenic emissions. These emissions are
not included in any emissions inventory.
Field measurement campaigns have been conducted from July 2013 through October
2014 at a total of forty-seven well sites. These campaigns involved measurements
of gas fluxes from wells and the immediate surrounding area as well as at control
sites near the wells. Eighty-one percent of the wells have positive flow rates (they
emit methane), 6.4% have flow rates indistinguishable from zero, and 13% have small
negative flow rates. The presence of other alkanes (ethane, propane, and n-butane)
in the wells with positive flow rates suggests that the methane emissions are not
produced from microbial processes and are thus of thermogenic origin. The isotopic
composition of the methane also indicates that the methane is of thermogenic origin.
A total of 23 wells have each been measured between 2 and 9 times. These multiple
measurements allow the variability in methane emission rates at these wells to
be assessed, including the degree to which the variations are associated with seasonal
changes. Overall the high-emitting wells (wells with flow rates greater than
104 mg/hr of CH4) displayed only a small degree of temporal variability compared
to low-emitting wells. There appears to be no relationship between the number of
times a well is measured and the variability in flow rate measurements. Preliminary
results indicate that season does not affect the emission rates for wells or for control
locations near wells. Land cover (forest, grassland, or wetland) is also explored as
a potential cause for variation among different well and control measurements. Preliminary
results indicate that land cover does not impact the emission rates for well
locations, but does affect the emission rates for control locations. These results could
impact the way future measurements are conducted and also serve to provide a more
accurate emission rate estimate.